This invention relates to photovoltaic devices useful for converting light into electrical energy, and more specifically to devices for converting solar energy into electrical energy.
The conversion of solar energy into electrical energy is one of the prime technologies which can have a major impact on the future energy requirements of the world as well as solving the impending global warming problems due to present increasing usage of carbon based energy sources. Development of efficient and low cost photovoltaic devices is key to significant utility of direct solar energy to electrical power conversion. The utility of inorganic semiconductor materials in photovoltaic cells to convert incident light energy into electrical energy is well known and constitutes a major article of commerce. Crystalline silicon is among the best-known inorganic semiconductors for photovoltaic devices and is widely employed. Other inorganics commonly noted for utility in photovoltaic devices include gallium arsenide, amorphous silicon and cadmium sulfide. For these semiconductors, the electronic structure is comprised of a valence band and a conduction band separated by an energy gap (band gap). An incident photon can excite the electron from the valence band into the conduction band of the semiconductor if the photon energy is higher than the band gap. The most common form of semiconductor junctions in photovoltaic cells is the p-n junction. An incident photon can excite the electron from the valence band into the conduction band of the semiconductor leaving mobile holes in the valence band and thereby generating electron-hole pairs. The electric field at the junction prevents the recombination, the holes move from the n position to the p position and electrons move in the opposite direction. If the junction is connected to electrodes, a current can be produced. Certain conjugated polymers and electroactive organic materials can also exhibit semiconductor-like properties and can also be employed in similar photovoltaic devices to convert solar energy to electrical energy.
Organic based photovoltaic devices offer specific fabrication and economic advantages over inorganic materials for photovoltaic applications if reasonable efficiencies can be achieved. These devices involve thin films of the active organic materials between metal electrodes of which at least one electrode is transparent to incident (e.g., sun) light. These organics (often referred to as light harvesting organics) include perylene, oligomeric thiophenes, phthalocyanines, triphenyl methane based compound, π-conjugated polymers, pyrromethene dyes, etc.
Phthalocyanine particles dispersed in a polymeric binder have been employed in photovoltaic devices. Minami et al. (J. Appl. Phys., 54(11), 6764-6766, November 1983) discloses the use of metal-free phthalocyanines dispersed in various polymers (ten different polymers are noted, including poly(vinyl acetate), polystyrene, polyacrylonitrile, poly(vinyl fluoride), poly(vinylidene chloride), and poly(vinylidene fluoride). These polymers were generally not high Tg or even exclusively amorphous. As the polymer is employed as a binder for particles, the polymer Tg is not specifically important. Film thicknesses employed were between 1 and 3 microns.
U.S. Pat. No. 4,175,981 to Loutfy et al. discloses the use of metal-free phthalocyanines dispersed in a polymeric binder (such as poly(vinyl acetate), Bisphenol A polycarbonate, polystyrene and styrene copolymers). The addition of a dye sensitizer (such as Coumarin dyes) is disclosed.
Brabec et al. (J. Appl. Phys., 85(9), 6866-6872, May 1999) disclose photovoltaic properties of a soluble derivative of poly (p-phenylene vinylene) and methanofullerene (donor-acceptor complex) embedded in a polystyrene matrix. The mixture was described as an interpenetrating polymer network.
U.S. Pat. No. 4,125,414 to Tang et al. describes a photovoltaic element comprised of a layer of an electrically insulating binder, a pyrylium-type dye salt and an organic photoconductor. Various polycarbonates were noted as the electrically insulating binder. Organic photoconductors employed included tritolylamine. These compositions are described as aggregate photoconductive materials, wherein a discrete discontinuous phase composed of a particulate co-crystalline phase of the binder and dye is dispersed in a continuous phase of the binder and photoconductor.
Yu and Heeger (J. Appl. Phys., 78(7), 4510-4515, 1 Oct. 1995) describe a phase separated polymer (poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene as a donor and cyano modified poly(phenylene vinylene) as an acceptor) for utility in photodetector and photovoltaic applications.
Shaheen (Synthetic Metals, 121, 1583-1584 (2001)) discusses low band-gap polymeric photovoltaic devices comprising thiophene-isothianapthene copolymers doped with soluble fullerene derivatives blended into PMMA to enhance film quality. The thiophene-isothianapthene copolymers had a molecular weight of 6000-8000 amu. The phase behavior of the copolymer and PMMA is not disclosed.
Despite the foregoing developments, it is desired to obtain compositions with improved characteristics for use in photovoltaic devices
All references cited herein are incorporated herein by reference in their entireties.